[feat]: Create `<pf-v6-overflow-menu>` element

[bennypowers]

Summary

Create `` to implement the PatternFly v6 Overflow menu component.

Responsive action bar that collapses items into a dropdown menu when space is constrained.

Prior Art

Source	Element	Status
PFE v4	--	--
cem	--	--
cockpit	--	--
chickadee	--	--

Requirements

• MUST implement PFv6 visual designs
• MUST provide end-user feature parity with `@patternfly/react-core` OverflowMenu
• MAY adjust element API to leverage web platform strengths
• SHOULD maintain CSS custom property theming compatibility
• SHOULD expose useful CSS shadow parts

Element-specific considerations

• React `breakpoint`: `sm`, `md`, `lg`, `xl`, `2xl` - below this breakpoint, items collapse into dropdown
• React `breakpointReference`: `window` or a CSS selector for container-relative breakpoint
• React sub-components: `OverflowMenuContent`, `OverflowMenuControl`, `OverflowMenuDropdownItem`, `OverflowMenuGroup`, `OverflowMenuItem`
• `OverflowMenuContent` - visible items; `OverflowMenuControl` - the "..." dropdown trigger
• Uses `` + `` for the overflow dropdown
• Use `ResizeObserver` or container queries to determine breakpoint; SSR-safe with `isServer` guard
• Overflow items must duplicate the label from visible items for accessible dropdown

Checklist

Implementation

• API per `.claude/ADVICE.md`
• CSS uses v6 tokens, logical properties, nesting
• `@summary` and JSDoc on element class

Demos

• Demos match patternfly.org naming/structure
• CSS custom properties match computed styles from patternfly.org (Chrome MCP)
• Visual parity verified via Chrome MCP

Tests

• Public API covered (attributes, properties, events, slots)
• `a11ySnapshot` assertions

Reviews

• `/review-api` passes
• `/review-demos` passes
• `/review-a11y` passes
• `eslint` and `stylelint` pass

Ship

• Add changeset
• PR targets `staging/pfv6` (NOT `main`)